1. Field of the Invention
The present invention relates to a method for driving an ink jet printing head and circuits of the same and more particularly to the method for driving the ink jet printing head and the circuits of the same in which a character or an image is printed on a printing medium such as paper, OHP (Overhead Projector) film or a like, by driving the ink jet printing head having a nozzle and by selectively jetting, from the nozzle, fine ink drops having a uniform size adjusted to meet desired printing resolution.
2. Description of the Related Art
In a conventional ink jet printing device, a printing dot is formed on a printing medium such as paper, OHP films or a like, by feeding, at a time of printing, a driving waveform signal to a pressure generating device including a piezo-electric actuator or a like disposed at a position corresponding to a pressure generating chamber of an ink jet printing head having the nozzle to cause a content volume of the pressure generating chamber filled with ink to rapidly change for jetting one ink drop from a nozzle. The ink jet printing device of this kind is widely applied to printing equipment such as a printer, plotter, copying machine, facsimile or a like.
In such ink jet printing devices as described above, since one dot is formed and then one image is created when one ink drop comes within range on the printing medium, a size of a printed dot diameter is approximately inversely proportional to an image quality. That is, in order to meet a recently increasing requirement for providing printing of a high image quality, it is necessary to form the printing dot having smaller diameter on the printing medium. The diameter of the printing dot (hereafter referred to as a "dot diameter") required for obtaining a smooth and excellent image of high quality being free from a feeling or sense of a "grain" at an area printed at a low density is considered to be not more than 40 .mu.m, more preferably not more than 25 .mu.m from a view point of discriminating capability of a human eye. In general, since the dot diameter is 2 to 2.5 times larger than that of the ink drop, to obtain the dot diameter of 40 .mu.m, the diameter of the ink drop has to be about 20 .mu.m. In this case, a total diameter of whole ink drops obtained by adding a volume of a main ink drop to that of a satellite ink which is a small ink drop formed secondarily at the rear of the main ink drop when being jetted from the nozzle is about 25 .mu.m.
On the other hand, it is known from experiments that a minimum value of a total diameter of whole ink drops jetted from the nozzle having a predetermined aperture diameter is almost equal to a diameter of the aperture itself (a diameter of the nozzle). Therefore, to obtain a total diameter of the ink drops being 25 .mu.m, the diameter of the nozzle must be not more than 25 .mu.m. It is, however, impossible to produce the nozzle that can be practically used having its diameter being not more than 25 .mu.m, without many difficulties. That is, the probability of occurrence of clogging in the nozzle increases, causing reliability and durability of the ink jet printing head to be very impaired. Because of this, a present lower limit of the nozzle diameter is about 25 to 30 .mu.m. Accordingly, in the conventional ink jet printing device, it is difficult to jet, in a stable manner, the ink drop having its diameter of not more than 25 .mu.m. Additionally, the conventional ink jet printing device has another problem in that, if the nozzle is designed to have its smaller diameter by simply aiming at making the ink drop finer, the ink drop having a maximum diameter of the whole ink drops enough to satisfy desired resolution cannot be jetted.
In an attempt to solve these problems, a method for driving the ink jet printing head is disclosed in, for example, Japanese Patent Application Laid-Open No. Sho55-17589, in which an ink drop being smaller in size than a nozzle diameter can be jetted by feeding an inverse trapezoidal driving waveform signal to a piezo-electric actuator to cause so-called "meniscus control" to be made immediately before jetting of the ink drop. In the method disclosed above, as shown in FIG. 8A, when jetting of the ink drop is not required, a meniscus 1 is positioned fitly at an aperture face 2a of a nozzle 2. When jetting of the ink drop is required, as shown in FIG. 8B, the meniscus 1 is retracted backward, from the position of the aperture face 2a of the nozzle 2 into an internal portion of the nozzle 2, by a driving waveform signal fed to the piezo-electric actuator, causing a content volume of a pressure generating chamber to be increased and, as a result, a shape of the meniscus becomes concave (this is called a "process of retraction"). Then, when the driving waveform signal causing the content volume of the pressure generating chamber to be decreased is fed to the piezo-electric actuator, as shown in FIG. 8C, an ink drop 3 is jetted (this is called a "process of pushing").
Moreover, another ink jet printing device is disclosed in Japanese Patent Publication No. Hei3-30507, in which a diameter of an ink drop jetted from a nozzle is changed by a variation in an amount of retracting movement (showing a "strength of retraction") of a meniscus 1 in the nozzle, occurring immediately before the jetting of the ink drop, or by a variation in timing of the retracting movement of the meniscus in the nozzle, occurring immediately before the jetting of the ink drop, which is caused by changes in a waveform of a driving waveform signal.
In the conventional method for driving the ink jet printing head or in the conventional ink jet printing device described above, since jetting characteristics including the diameter of the ink drop or a falling speed of the ink drop from the nozzle or a like are changed depending on the amount of the retraction of the meniscus in the nozzle occurring immediately before the jetting of the ink drop, the change in the diameter of the ink drop is more responsive to dispersion in dimensions of parts or external perturbations, compared with a case where the ink drop is jetted without the meniscus control.
Also, in the conventional method for driving the ink jet printing head or in the conventional ink jet printing device described above, the meniscus is retracted and the ink drop is jetted when the driving waveform signal is fed to the piezo-electric actuator to cause the content volume of the pressure generating chamber to be increased or decreased. However, the piezo-electric actuator responds not faithfully to an applied driving waveform signal but it responds to the signal, to some extent, in a vibrating manner. Since the content volume of the pressure generating chamber is changed whenever the piezo-electric actuator is vibrated, the meniscus 1 makes a reciprocating movement in the nozzle immediately before the jetting of the ink drop, as described in the above Japanese Patent Publication No. Hei03-30507. Due to adverse effects caused by a jetting history, crosstalk, use environments or a like, the retracting amount of the meniscus in the nozzle cannot be constant, even if the meniscus is retracted in a same nozzle and, as a result, the total diameter of the ink drop is changed. Therefore, the conventional method for driving the ink jet printing head and the ink jet printing device have problems in that the ink drop having a desired small diameter is not jetted successfully, a formation of the ink drop becomes very unstable and a failure in jetting the ink drop occurs.